1. Field of the Invention
Wood screws belong to the most widely used fasteners in many different applications. For instance, it is often required that a wood screw be capable of fixedly holding a hardware such as a hinge wing or railway clamp. Another requirement often raised is that the specific axial pressure active under the screw head of a tightened screw be as low as possible while securing a firm hold. Another requisite Is that the screw be capable of being applied without the need for countersinking the head as this requires an extra machining of at least one of the parts being joined. By the same token, it is often required that the screw positioned in place be as unobtrusive as possible on the surface to avoid the danger of injury, for instance, to animals in zoo structures, to children and to the general user.
2. Description of the Prior Art
Various types of screws are known which are directed to satisfy one or more of the above requirements. For instance, it known from Italian Patent 475002 Manfroni! to provide a railway clamp fastening wood screw which includes a cylindric portion of the shank of the screw above the threaded portion of the shank and just below the head. The cylindric portion is closely compatible with a bore in the railway clamp through which the screw passes into the wooden tie to hold the clamp down. Viewed from the standpoint of the present invention, the screw has the drawback that the specific pressure under the screw head is relatively high and that an axially upwardly projecting portion is required for engagement with a torque applying tool, in this case a socket wrench. This type of screw therefore would be unsuitable for applications where the specific pressure must be low, the fastening force high and the surface head as flush as possible to avoid or at least substantially reduce the danger of injury to a child or an animal.
The torque tool engaging projection is known to be replaced with various cutouts compatible with tools such as screwdrivers and a variety of these exists, from plain straight grooves as in Canadian Patent 27,387 Rogers!, to more complex sockets such as shown e.g. in U.S. Pat. No. 5,249,882 Nagoshi et al.! Canadian Patent 400,454 Purtell!, Canadian Patent 1,248,374 Rockenfeller et al.! or Published Canadian Patent Application 2,094,478 Goss!.
Prior art screws are usually unsuitable for use with power tools, where the screw must withstand a relatively high torque while a low specific pressure under the head is required when the fastener is firmly in place. The screws having convexly shaped heads project above the surface of the workpiece often in obtrusive and potentially dangerous way which may cause injury.